the formula for this is time= speed in mph divided by the distance
example 100 mph divided by 100 miles =1hour
there is no average speed as it will be set by speed restrictions so if the 100 miles was all 50 mph speed zone it would take 2 hours.

On your Saturn it is likely a bad or loose fuel cap. Is there a lot of rust around the filler neck? If so try carefully removing it and then wiping the rubber gasket of the fuel cap with a bit of grease. Once you reset the code you need to drive the Vue for a a cycle in order to reset the sensor even though you have cleared the OBD code with a code reader.